Fluid circulation and diagenesis in an off-axis hydrothermal system: The Mariana Mounds

• Main Author: Wheat, Charles Geoffrey
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-1990
• Subjects: Geochemistry
• ISBN: 9798208585597

This dissertation focuses on determining the circulation pattern and the effects of fluid flow on the evolution of the crust in an off-axis hydrothermal system. Results from this study allow chemical fluxes to be estimated, thereby, providing more insight to the global significance of chemical fluxes from off-axis hydrothermal systems. The area of study is the Mariana Mounds, which is an off-axis hydrothermal system located west of the Mariana Spreading Center overlying 3 my crust. The work presented in this dissertation is based on fitting chemical profiles of pore waters from fourteen piston and thirty-three ALVIN cores, the latter are 20 to 30 cm cores taken in specific features, to models derived from a general advection-diffusion-reaction equation. The pattern of fluid circulation in the Mariana Mounds is different from that in the Galapagos Mounds. In contrast to the southern section of the Galapagos Mounds, most of the fluid inputs to basement result from flow through faults and outcrops, presumably through the 500-800 m scarp located 2-4 km to the east and southeast. Downwelling of fluid through the sediments provides less than 2% of fluid inputs to basement. Downwelling was observed in two cores, at speeds of 1.5 and 20 mm/y. An average downwelling speed of 1.5 mm/y in areas with low heat flow is required to supply 2% of the fluid to basement. Upwelling is evident at rates of mm/y to km/y. The fluid in basement is oxygenated and undersaturated with respect to amorphous silica, resulting in an increase in the extent of degradation of organic matter and dissolution of amorphous silica in areas with upward flow relative to areas without flow. If upwelling occurs at speeds greater than 10 cm/y, then amorphous silica may be completely dissolved in the basal sediments, and may impeded with the later formation of a chert layer. Dissolution of amorphous silica in the Mariana Mounds results in a flux of silica that is at least twice as large as the expected flux from a similar area without flow. Projected on a global scale, the flux of silica from off-axis hydrothermal systems is at least 25% of the flux of silica from rivers. Water-rock interactions in off-axis hydrothermal areas can result in chemical fluxes that rival fluxes from riverine inputs, even if the chemical anomaly is at the limit of detection.